Phenolic-aldehyde resins are generally highly brittle low impact strength materials of limited compatibility with other plastics. Plasticizers have been employed with these materials to increase the flexibility thereof, but increased flexibility has resulted in the diminution of other properties and processing difficulties. For example, to obtain any significant degree of flexibility relatively large amounts, often equalling or exceeding the amount of phenol-aldehyde resin, of glycols or glycerine must be used. These large amounts of glycols or glycerine diminish the product strength and induce sweating out of the glycol or glycerine during the heat curing cycle. Furthermore, plasticization by crosslinking with unsaturated polymers such as polyvinylbutyral or other compatible elastomers has not been wholly satisfactory because of the lowered heat and solvent resistance of the resulting products. These products, while possessing improved impact strength, still lack suitable flexibility. Likewise, the use of unsaturated oils such as tung oil as resin modifiers, even in substantial amounts, fails to provide sufficient flexibility to the phenol-aldehyde resin product.
The inclusion of finely divided fillers such as wood flour and walnut shell flour in these products adversely affects their heat resistance and dimensional stability and does little to improve impact strength. When the molded phenol-aldehyde product contains long fibered cellulosic fillers such as paper flock, cotton flock, or sisal fiber it exhibits adequate impact strength but remains deficient in heat resistance and dimensional stability. The use of these long fibered cellulosic fillers also adversely affects the surface qualities of the molded product, thereby rendering it unsatisfactory for many applications.
The prior art has attempted to remedy some of these aforediscussed drawbacks by utilizing phenol-aldehyde articles comprised of oil modified phenol-aldehyde resins, such as cashew nut shell oil modified phenol-aldehyde resins; a copolymer of butadiene-acrylonitrile; and certain silicate fillers such as diatomaceous earth. However, these articles are still unsatisfactory as they exhibit relatively low heat resistance and significantly reduced rigidity which results in a lowered heat distortion temperature. This poor heat resistance and lowered heat distortion temperature is due to the utilization of an oil modified, such as cashew nut shell oil modified, phenol-aldehyde resin and the resultant lowered crosslink density of the resin matrix in the molded article.
There thus exists a need for a molded product made from a phenol-aldehyde resin which has good impact strength, good high temperature rigidity, high heat resistance, good dimensional stability, good surface appearance, high flexural modulus, and good heat stability, i.e., the ability to retain its properties upon exposure to heat. The present invention provides such a phenolic-aldehyde molded article and a molding composition for producing such an article.